A number of methods have been used in the past and are still presently in use for coating printed circuits. One of the most widely used methods is screen printing wherein a surface area of a board to be coated is defined by an opening in the screen device. Coating material is applied to the screen and is urged through the opening by the wiping action of a squeegee drawn across the screen. The amount of material deposited on the surface is determined by screen dimensions, material rheology and the speed of the squeegee traversing the screen. Whereas the screen method has the advantages of being accurate, fast and repeatable, disadvantages reside in the need to provide a specific screen for a specific board size, the complicated setup needed to establish machine parameters and the limited life of a screen.
A second method in popular use is curtain coating. In practice, a web of liquid is caused to fall from a metering device in a known and controlled geometry. The web is situated transverse to a conveyor or similar handling mechanism. The workpiece passes beneath the web and intersects the falling liquid and collects a controlled amount of falling liquid along an upper surface of the workpiece. Whereas curtain coating has the advantages of high productivity and minimal setup when changing from one board size to another, disadvantages reside in the fact that the equipment is not easily turned off and on again for short-run work, the equipment is relatively expensive in terms of capital required and the technique requires a large charge of material for proper operation. When thin coatings are desired, there is a tendency to skip and thereby not completely and properly cover the surface to be protected.
A third method of application is electrostatic spray which requires an electrically conductive material. A voltage is applied between the spray device and the surface being coated. An electric field created by the applied voltage causes the coating material particles to be accelerated and attracted to the surface being coated. Whereas advantages reside in the ability to completely coat the surface with negligible setup, the disadvantages are: high initial costs; tendency to waste material due to overspray; potential for electrical discharge and operator hazard to high voltage in the work area.